The conventional thermal recording media may include those which each has a thermosensitive recording layer disposed on a substrate as may be made of paper. In the thermosensitive recording layer are contained a color-forming agent and a color-developing agent causing color development by contact with the color-forming agent, and color developed images can be obtained by heating with a thermal pen, thermal head, or other means.
The thermal recording media of this type have been widely used as the recording media for facsimiles, various calculators, medical instruments, computers, thermal copying machines, and printers of other various instruments.
The advance of various office instruments and diversification of their applications are making rapid progress, and there has been a demand for the development of thermal recording media according to the respective requests. For example, thermal recording media suitable for the speed improvement of recording instruments are required to give high-density and distinct images recorded even with very low printing energy. It has become recognized that the study of various substrates as well as thermosensitive recording layers is needed for complying with the requirement. Therefore, cases are gradually increasing where synthetic paper or synthetic resin films are used in place of conventional natural paper.
As the means of fitting a decrease in printing energy involved in the speed improvement of printing, for example, JP-A-63299976 teaches the use of synthetic paper with a density of 0.9 g/cm.sup.3 or lower, e.g., 0.77 g/cm.sup.3, as a substrate for the sensitivity improvement of thermal recording materials. JP-A-05058028 and JP-A-05058029 teach, for the purpose of obtaining high sensitive thermal recording materials, the use of a polypropylene substrate with a density of from 0.6 to 0.8 g/cm.sup.3, on which surface with a Beck smoothness of 800 seconds or higher a recording layer is disposed, and the use of a microvoid-containing white-colored polyester as a substrate, respectively. In addition, JP-A-02302887 teaches laminating a plastic film with a density of from 0.5 to 0.8 g/cm.sup.3 on the base layer of paper in thermal recording sheets for the purpose of improving the degree of close contact with recording heads and preventing the curling of sheets. Furthermore, JP-A-09095056 teaches, for the purpose of obtaining high sensitive thermal recording materials, a synthetic resin sheet with a density of from 0.2 to 0.5 g/cm.sup.3.
These means, however, cannot have sufficient heat sensitivity and satisfactory delamination resistance.